Current hydrocarbon seismic exploration methods mainly rely on active seismic sources, i.e. sources controlled by the survey operator, to generate seismic energy. The recorded seismic signals from such sources are used to form images of reflector locations and to derive physical properties of the subsurface. However, current active source seismic acquisition techniques are unable to provide reliable and useful data in the low-frequency regime below roughly 6-8 Hz. (The more powerful explosions necessary to generate lower frequencies are not acceptable seismic survey activity because of environmental and societal impact.) The lower frequencies that are missing from this data contain information on the background trends in the rock properties. In addition, this information is important for stabilizing and steering pre-stack seismic inversion to the correct solution, and for deriving absolute rock properties in the subsurface. This technical difficulty (called the low frequency lacuna) renders advanced gradient-based seismic data inversion/imaging algorithms useless in problem data areas where conventional seismic data processing methods notoriously fail.
Current practices for addressing the low frequency lacuna involve estimating some portion of the missing spectrum of the earth model (generally information at the very bottom 0-2 Hz) and might include moveout-based velocity analysis (Liu, “An analytical approach to migration velocity analysis”, Geophysics 62, 1238-1249 (1997)), or reflection tomography methods (Foss, et. al., “Depth-consistent reflection tomography using PP and PS seismic data”, Geophysics 70, U51-U65 (2005)), or constructing subsurface models from seismic interpretation of the major reflectors observed in the 2D or 3D seismic reflection data. Unfortunately this approach fails if a satisfactory imaging velocity cannot be found in the pre-stack data.
What is needed is a source of data filling the low frequency lacuna, and a way to exploit such data to supplement the predictions and analyses made from seismic data that are so useful in petroleum exploration and production.